chapter 2: design and synthesis of coumarin based...

Chapter 2: Design and synthesis of Coumarin based sensitizers for dye-sensitized solar cells (DSSCs)

Synthesis of novel colorants for dye-sensitized solar cells and use of greener protocols for heterocyclic synthesis Page 39

2.1 INTRODUCTION

Coumarin chromophores have been extensively investigated as suitable materials in

hi-technological fields including electronic or photonic applications (Christie and

Lui 2000; Ayyangar et al.1991; Moylan 1994; Fischer et al. 1995) as fluorescence

probes, nonlinear optical materials, solar energy collectors and charge-transfer

agents. They have also been successfully derivatized to find use as significant

organic fluorescent materials (Christie 1993; Krasovitskii and Bolotin 1988). Their

commercial value and applicability in versatile fields can be mainly attributed to

their inherent photochemical features, efficient light emission properties, relative

ease of synthesis, good stability and solubility. The fluorescent dyes based on

coumarin show absorption in the UV region and emission of blue light (Barton and

Davidson 1974; Moeckli 1980). The nature of these dyes can be well related with

the structural changes in the coumarin moiety. For example, the fluorescent

coumarin dyes usually contain an electron-accepting group in the 3-position and

electron-donating group at the 7th position. The substitution at 7th position mainly

comprise of amino, hydroxy and methoxy groups. The increase in conjugation

results in deepening of colours by means of bathochromic shift whereas

incorporation of a benzo ring fused at the 5,6 position also confers good colour.

Such properties of coumarin based dyes encourage exploration of various structural

features by suitable designing so as to derive a range of colorants with varied

properties.

Dye-sensitized solar cells (DSSCs) have attracted significant attention as low-

cost devices for the photovoltaic conversion of solar energy compared with the

conventional solid p–n junction photovoltaic devices (Gratzel 2001; Gratzel



2004). Ruthenium poly-pyridyl complexes are the most widely used sensitizers for

DSSCs, achieving power conversion efficiencies (η) >10% (Wang et al. 2003) and

good long-term stability (Liang et al. 2007). Although the most efficient sensitizers

to date are ruthenium complexes, metal free organic dyes have been attracting

attention because of their ease of synthesis, high molar extinction coefficient,

tuneable absorption spectral response from the visible to the near infrared

(NIR) region, environmental friendliness and inexpensive production

techniques.

Among the metal-free organic dyes studied in DSSCs, coumarin-based dyes are

promising sensitizers owing to their good photo-response in the visible region, good

long-term stability under one sun soaking (Wang et al. 2007), and appropriate

lowest unoccupied molecular orbital (LUMO) levels matching the

conduction band of TiO2. Some coumarin sensitizers as donor groups have reached

efficiencies of upto 8.2 % values comparable to the standard N719 sensitizer (Hara

et al. 2001; Hara et al. 2003; Hara et al. 2003a; Hara et al. 2003b; Hara et al. 2005;

Hara et al. 2005a). The importance of coumarin is understood from the fact that on

replacing the coumarin moiety with N, N-dimethylaminophenyl (DMA) donor

group, (Hara et al. 2005b; Kitamura et al. 2004) a significant hypsochromic shift of

the maximum absorption peak is observed indicating that the coumarin is a

stronger donor than the DMA group.

In order to obtain a red shift, it is important to extend π-conjugation which is usually

done by extending the methine unit (–CH=CH–) of the molecule. However, such an

extension by more double bonds would increase the instability of the dye molecule,



owing to the possibility of isomer formation. Also, report by K. Hara (Hara et al.

2001; Hara et al. 2003) indicated that among a series of coumarin dyes with

extended methine units, the highest values of Jsc and efficiency is obtained with two

methine units (n=1 in figure 2.1). The dyes with methine units greater than 2 give

reduced values of Jsc and Voc mainly due to the H-aggregation of the dye due to

strong interactions between dye molecules on the TiO2 surface.

Figure 2.1 Structure of coumarin dyes (n is the no. of methine units)

Another important way of increasing conjugation is by incorporation of benzene and

thiophene rings as the linker unit in the dye structure. This would simultaneously

extend π-conjugation and improve the stability of the dye molecule relative to

the dyes, which have a long methine chain unit. It has been observed that

introduction of thiophene moieties improves the solar cell performance mainly by

broadening the absorption spectra thereby resulting in a large photocurrent and also

relatively lowers the positions of the LUMO levels of the dyes (see figure 2.2).

As the number of thiophene units is increased, the LUMO levels are much lowered

due to increased π-π stacking leading to insufficient driving force for electron

injection (Hara et al. 2003a; Hara et al. 2005a).

N O O

CN

COOHn


Synthesis of novel colorants for dye-sensitized solar cells and use of greener protocols for heterocyclic synthesis 2

Figure 2.2 Structure of coumarin dyes (n is the no. of thiophene units)

Herein, we designed molecules based on coumarin moiety so as to be suitable for

application in dye-sensitized solar cells. One of the major requirements of molecules

for dye-sensitized solar cells is their broad absorption in the visible region. In order

to shift the absorption at longer wavelength in coumarin based fluorescent dyes, we

decided to strengthen both the donor and acceptor at 7 and 3- position. At the 7th

position, we took N,N-diethyl amino group as a donor. The acceptor was

incorporated at position 3 by means of conjugating bridge containing halogen atom.

Figure 2.3 Structural features of the synthesized dye

N O O

Cl

H3C

H3C

CN

COOH

LINKER DONOR

ACCEPTOR

N O O

nS

S

COOHNC



We also prepared coumarin based dyes by replacing N,N-diethylamino group at 7-

position by julolidine group (figure 2.4) which is strong electron donor and can

influence the strength and overlap of a nitrogen donor orbital with a conjugation

system.

Figure 2.4 Coumarin with julolidene

donor group

The concept of lateral anchoring has been explored recently by many scientists

wherein the anchoring carboxylic group is separated from the electron acceptor

groups of the sensitizer. The lateral anchoring provides several alternatives for

conventionally used cyanoacrylic acid groups as the acceptor end. This would not

only result in possibility of various new structures but also can help in overcoming

the restriction of HOMO-LUMO fine tuning. The functioning of these dyes is

expected to occur via injection of electrons from acceptor group rather than

anchoring group to conduction band of titanium dioxide (TiO2). However, further

modifications and also an in-depth study is required so as to make these dyes more

efficient than several other conventional cyanoacrylic acid group containing dyes.

To mention few reports on such type of dyes, Sun et al adopted this strategy for the

first time and were successful in extending the absorption spectra of the sensitizers

(Figure 2.5) towards the NIR region (Hao et al. 2009). Inspite of several other

reports, recently Sun et al reported lower efficiencies with lateral anchoring group



(Figure 2.6) wherein they explained attributed this behaviour to the electron

recombination process due to the proximity between donor group and titanium

dioxide surface (Hao et al. 2012). Some examples of such dyes with concept of

lateral anchoring group is provided in figures 2.5 and 2.7.

A (λmax = 610 nm) B (λmax = 638 nm)

Figure 2.5 Structure of near absorbing dyes with lateral anchoring groups

C (Efficiency = 7.0%) D (Efficiency = 2.7%)

Figure 2.6 Example of decrease in efficiency with incorporation of lateral anchoring groups

Hence, we attempted to synthesize dyes with lateral anchoring carboxylic acid group

that is not directly attached on the donor coumarin group. In this regards, chloro

group could be replaced for the incorporation of alkyl chains containing carboxylic

acid end groups (as shown in figure 2.7). After incorporation of anchoring group, we



substituted the aldehyde of coumarin group with malononitrile. We aim to improvise

in this direction with further modifications and study, so as to obtain dyes with better

efficiencies.

Figure 2.7 Scope of incorporating anchoring groups in our present dyes



2.2 RESULTS AND DISCUSSION

2.2.1 Synthesis of intermediates and coumarin sensitizers for dye-sensitized solar

cells

Keeping in mind all the aspects discussed in introduction section, we synthesized

colorants suitable for application in dye-sensitized solar cell with benzene, thiophene

and furan rings in the methine chain as linker unit as shown in schemes 2.1-2.5. The

sensitizers [10a-10c] were synthesized by condensation of a formylcoumarin

compound [4] with cyanoacetic acid [8] or active methylene compounds containing

cyanocarboxylic acid groups [9a-9c] in ethanol using piperidine. The

formylcoumarin compound [4] was prepared in two steps starting from DEMAP

aldehyde [1] and ethyl cyanoacetate [2] in ethanol using piperidine to obtain acetyl

coumarin intermediate [3]. This was followed by reaction with DMF/POCl3 that

introduces a chloro group in conjugation with the aldehyde group. The active methyl

containing compounds [9a] were synthesized in two steps including formylation of

compounds [6] with DMF/POCl3 and condensation with cyanoacetic acid [8]

whereas, in case of [9b], only the latter step was applied. The fused coumarin

compound was prepared in six steps involving reaction of m-anisidine [11] with

bromochloropropane to form methoxy derivative of julolidene [12]. This was

followed by its demethylation using HI and formylation using DMF/POCl3 which

was finally cyclized with ethylacetoacetate to obtain acetyl fused coumarin

derivative [16]. This acetyl derivative was again subjected to formylation and

knovenagel condensation with cyanoacetic acid to form final compound [18]. The

coumarin aldehyde [4] was further subjected to replacement of chloro group with

side chain thiocarboxylic acid anchoring groups. This was followed by condensation



with malononitrile [22] to obtain the final chromophore [23]. The chromophores and

intermediates were characterized by FT-IR, 1H NMR, 13C NMR, mass spectrometry.

The UV-visible absorption and emission spectra were recorded for all these

synthesized derivatives. The physical properties of these colorants are summarized

in Table 2.1.

Scheme 2.1: Synthesis of 3-chloro-3-(7-(diethylamino)-2-oxo-2H-chromen-3-

yl)acrylaldehyde [4]

Scheme 2.2: Synthesis of active methyl compounds

CN

COOH

[5]

Ammonium acetate

Glacial CH3COOH

SH3C

H

O

H3C

H

O

SH3C

SH3C HOOC

CN

DMF / POCl3

SH3C

H

O

+Ethanol

H3C

CN

HOOC

[6]

[7]

[6]

[8]

[9b]

[9a]

HN

O

+0-5 oC

N

CHO

OHH2C

CH3

O

OCH2CH3

O

N O O

O

CH3

Ethanol / Piperidine

Ref lux

+

[1] [2] [3]

N O O

O

CH3DMF / POCl3 N O O

Cl

CHO

[3] [4]

0-5oC



Scheme 2.3: Synthesis of sensitizers [10a-10c]

Ethanol

Reflux

N O O

Cl

CHO

[4]

NC

COOH

[8]

+N O O

Cl

[10a]

CN

COOHPiperidine

[10b]

H3C

CN

HOOC

[9a]

N O O

Cl

H3C

H3C

CN

COOH

N O O

Cl

CHO

[4]

+

Ethanol

Reflux

Piperidine

SH3C

HOOC

CN

N O O

Cl

H3C

H3C

S

COOH

NC

N O O

Cl

CHO

[4]

+

[9b]

[10c]

Ethanol

Reflux

Piperidine



Scheme 2.4: Synthesis of (2Z,4Z)-5-chloro-2-cyano-5-(11-oxo-2,3,5,6,7,11-hexahydro-1H-pyrano[2,3-f]pyrido[3,2,1-ij]quinolin-10-yl)penta-2,4-dienoic acid [18]

H2N OCH3 N O

CH3Br Cl2

[11]

+

[12]

N OHN OCH3

[12]

[13]

N OH

CHO

N O

CH3

O

O

CH3

O

OEtO

[14] [16]

Ethanol+

Piperidene[15]

N O O

Cl

CHO

+ C

[17]

POCl3

HN

O

N O

CH3

O

O

[16]

N OH N OH

CHO

[13]

C

[14]

POCl3

HN

O

+

N O O

Cl

CHO

N O O

Cl

CN

COOHCN

COOH

Ethanol+

Piperidene[18][17]



Scheme 2.5: Synthesis of (Z)-3-((4,4-dicyano-1-(7-(diethylamino)-2-oxo-2H-

chromen-3-yl)buta-1,3-dien-1-yl)thio)propanoic acid [23]

Table 2.1: Physical properties of Coumarin based sensitizers

Dye No. Molecular Formula Molecular weight

Yield (%)

10a C19H17ClN2O4 373 89

10b C27H23ClN2O4 475 59

10c C25H21ClN2O4S 481 45

18 C21H17ClN2O4 397 54

23 C22H21N3O4S 423 64

N O O

ClCHO

H3C

H3CEthanol / Triethylamine

Reflux

+ HSCOOH N O O

SCHO

H3C

H3C

COOH

N O O

S

H3C

H3C

COOH

CN

CN+

[4] [20] [21]

[22] [23]

N O O

SCHO

H3C

H3C

COOH

[21]

Ethanol / Piperidene

RefluxCN

CN



2.2.2 Photophysical properties of Coumarin dyes

The linear absorption spectra of the newly synthesized sensitizers were measured for

concentrations of 1×10-3 M in chloroform. The path length of the cell was 1 cm

whereby the influences of the quartz cuvette and the solvent have been subtracted.

The dyes 10a-10b show good absorption in visible region especially in the region of

400-500 nm whereas the absorption of dye 10c extends even in the range of 500-600

nm (Figure 2.8). This could be possibly owing to the better conjugation

characteristics incorporated by use of thiophene linker.

In case of fused amino coumarin dye 18, the absorption is mainly observed between

400-600 nm (Figure 2.9). The sensitizer 23 wherein the chloro group was replaced

with carboxylic acid containing thiol group, showed a very broad absorption starting

from 200 nm and extending till 600 nm. The broad nature of absorption could also

be attributed to the better charge separation induced by presence of stronger

electron-withdrawing cyano groups.



Figure 2.8: UV-Visible spectra of synthesized colorants 10-10c in chloroform

DYE 10a

DYE 10b

DYE 10c



Figure 2.9: UV-Visible spectra of synthesized colorants 18 and 23 in chloroform

DYE 23

DYE 18



2.2.3 Thermogravimetric analysis

We also tested the thermal stability of dyes by thermo gravimetric analysis (TGA)

carried out in the temperature range 25-600 °C under nitrogen gas at a heating rate

of 10 °C min-1. The TGA curves revealed that most of the dyes hold extremely good

thermal stability with majority of dyes showing stability above 250 °C as revealed in

Table 2.2. The sensitizer 10a showed the best thermal stability amongst all. The

molecule 10b containing benzene bridge as conjugation bridge showed better

stability than 10c that contains thiophene ring. However, colorant with fused

coumarin ring 18 showed lower stability which might be because presence of

alicyclic ring. The higher value of thermal stability is desirable in high-technological

applications like dye-sensitized solar cells.

Table 2.2: Thermal stability of coumarin based sensitizers

Dye No. Temperature stability (°C)

(at 3% percentage decomposition)

10a 309

10b 295

10c 267

18 275



2.3 EXPERIMENTAL 2.3.1 Materials and equipments

All the solvents and chemicals were procured from S D fine chemicals (India) and

were used without further purification. The reactions were monitored by TLC using

0.25 mm E-Merck silica gel 60 F254 precoated plates, which were visualized with

UV light. UV – Visible absorption spectra were recorded on Spectronic genesis 2

spectrophotometer instruments from dye solutions (~ 10-3 M) in chloroform. The 1H

NMR spectra were recorded on 400 MHz on Varian mercury plus spectrometer.

Chemical shifts are expressed in δ ppm using TMS as an internal standard. Mass

spectral data were obtained with micromass-Q-Tof (YA105) spectrometer.

Elemental analysis was done on Harieus rapid analyzer. Melting points measured

and thermogravimetric analysis was carried out on SDT Q600 v8.2 Build 100 model

of TA instruments.

The linear absorption spectra of the newly synthesized push pull chromophores were

measured for concentrations of 1×10-3 M in chloroform in a cell of 1 cm path length

whereby the influences of the quartz cuvette and the solvent have been subtracted.

2.3.2 Synthesis of key intermediates and compounds

2.3.2.1 Preparation of 3-acetyl-7-(N, N-diethyl) amino benzopyan-2-one [3]

In a 500ml three-necked round bottomed flask, 4-(N, N-diethyl) amino

salicylaldehyde [1] (19.3 g, 0.1 mole) was dissolved in 25 ml of ethanol. This was

followed by the addition of ethyl acetoacetate [2] (13.0 g, 0.1 mole) and piperidine

(0.5ml) under ice cold conditions. The solution was stirred at room temperature for

half an hour and refluxed for 1 hour. The product separated out as pale yellow



crystals from the reaction mixture was filtered and washed with ethanol. The

compound was recrystallised from ethanol.

Yield = 22 g (86%); M.P. = 154 °C (lit. 150-152 °C, Lin et al. 2009).

2.3.2.2 Synthesis of 3-chloro-3-(7-(diethylamino)-2-oxo-2H-chromen-3-yl)

acrylaldehyde [4]

In a three necked 100ml round bottom flask fitted with a mercury sealed stirrer,

addition dropping funnel topped by calcium chloride guard tube and reflux

condenser also topped by calcium chloride guard tube. N,N-dimethyl formamide

(2.68g, 2.82ml, 0.036 moles) was taken and cooled to 0-5 °C with stirring. To the

above solution phosphorous oxychloride (3.79g, 2.30ml, 0.024moles) was added

drop wise maintaining the temperature of the reaction mass at 0-5 °C. The Vilsmeier

complex so formed was stirred for further 15 minutes and [3] (4g, 0.015moles) was

added in lots (15-25 minutes) to the complex. The reaction mixture was stirred at 0-5

°C for 3 hrs and then allowed to attain room temperature. After that the contents of

the flask were heated at 80-85 °C in a water bath for 6-7 hrs. Subsequently the

reaction mass was cooled to room temperature and poured in to crushed ice with

stirring and deep external cooling, the clear solution obtained was neutralized with

sodium carbonate to pH 7-8, by keeping the temperature below 10 °C. The product

separated was brick solid. This was filtered, washed with ice cold water and dried at

50oC. The compound was crystallized from ethanol as brick red powder.

Yield = 3.0 g (65%); M.P. = 198 °C; 1H NMR (CDCl3, TMS): ): δ 10.25 (s, 1H),

8.40 (s, 1H), 7.70 (m, 2H), 7.42-7.38 (m, 1H), 6.65-6.60 (m, 1H), 6.45 (s, 1H), 3.45

(m, 4H), 1.30-1.25 (t, 6H); 13C NMR (CDCl3, ppm) δ 192.56, 152.85, 145.36,

144.98, 131.06, 126.28, 113.14, 110.01, 108.45, 96.59, 45.26, 12.54.



2.3.2.3 Synthesis of 5-methylthiophene-2-carbaldehyde [6]

A three necked 100ml round bottom flask is taken fitted with a mercury sealed

stirrer, addition dropping funnel topped by calcium chloride guard tube and reflux

condenser also topped by calcium chloride guard tube. N, N-dimethyl formamide

(11.0 g, 12 ml, 0.153 moles) was taken and cooled to 0-5°C with stirring. To the

above solution phosphorous oxychloride (23 g, 14 ml, 0.153 moles) was added drop

wise maintaining the temperature of the reaction mass at 0-5 °C. The Vilsmeier

complex so formed was stirred for further 15 minutes and solution of 2-methyl

thiophene [5] (10g, 9.3 ml, 0.102 moles) in DMF (7g, 9 ml, 0.1 moles) was added in

lots (15-25 minutes) to the complex. The reaction mixture was stirred at 0-5 °C for

1.5 hrs and then allowed to attain room temperature. After that the contents of the

flask were heated at 65-70 °C in a water bath for 11 hrs. Subsequently the reaction

mass was cooled to room temperature and poured in to crushed ice with stirring and

deep external cooling, the clear solution obtained was neutralized with sodium

hydroxide to pH 7-8, by keeping the temperature below 10 °C. The reaction mass

was extracted using ethyl acetate. The ethyl acetate layer was washed with water and

then subjected to evaporation under vacuum using rotary evaporator to obtain an oily

viscous brown-colored liquid with 65% yield.

2.3.2.4 Synthesis of (Z)-2-cyano-3-(5-methylthiophen-2-yl)acrylic acid [9a]

In a three-necked 100 mL round bottomed flask, 5-methylthiophene-2-carbaldehyde

[6] (7 g, 0.05 moles) in absolute ethanol (35 ml, 5 vol) was taken to which

cyanoacetic acid [8] (4.72 g, 0.05 moles) and piperidine (2–3 drops) were added

with stirring and then the mixture was refluxed for 6 hrs. The precipitated solid was



filtered, washed with ethanol and dried give 85% yields and used for further reaction

without purification.

2.3.2.5 Synthesis of (Z)-2-cyano-3-(p-tolyl)acrylic acid [9b] was synthesized by the

same procedure as that for compound [9a]. M.P. = 210 °C

2.3.2.6 Synthesis of 5-chloro-2-cyano-5-(7-(diethylamino)-2-oxo-2H-chromen-3-yl)

penta-2,4-dienoic acid [10a]

In a three-necked 100 mL round bottomed flask, 3-chloro-3-(7-(diethyl amino)-2-

oxo-2H-chromen-3-yl)acrylaldehyde (1g, 3.2 mmol) was taken in absolute ethanol

(10 mL, 10 vol). This was followed by the addition of cyanoacetic acid (0.55 g, 6.5

mmol) and piperidine (3-4 drops) and the reaction mixture was vigorously stirred at

reflux temperature for 4 hrs. The progress of the reaction was monitored by TLC.

After completion of the reaction, the reaction mass was added to cold water and

product was extracted using ethyl acetate. The ethyl acetate layer was washed with

water and then subjected to evaporation under vacuum using rotary evaporator to

obtain product [10a]. The crude product was further purified by silica gel column

chromatography using toluene:ethyl acetate system (6:4) as eluent system.

Yield = 1.0g (89%); M.P. = 242 °C.

Analysis of dye [10a]:

A. Mass spectra of the compound showed molecular ion peak at m/z = 373

which corresponds to molecular weight of [10a] = 372; msms showed ion

peak at m/z = 337 which corresponds to mass of fragment obtained after

removal of chloro group.

B. IR spectra of [10a]

Presence of broad band at 3383cm-1(s) indicating O-H stretching



Presence of band at 3000-3100cm-1(s) indicating aromatic C-H

Presence of band at 2210 cm-1(s) indicating C=N stretching

Presence of band at 1702 cm-1(m) indicating C=O stretching

Presence of band at 1170 cm-1(s) indicating C-N stretching

Presence of band at 767 cm-1(m) indicating C-Cl bending

C. The compound was further confirmed by which showed following signals

[10a]

1H NMR (CDCl3, 300 MHz): δ (ppm) 8.44 (s, 1H, aromatic CH); 8.18-8.15

(m, 1H, aromatic CH); 8.04-8.01 (d, 1H, vinylic CH); 7.64-7.62 (d, 1H,

aromatic CH); 6.79-6.76 (d, 1H, vinylic CH); 6.59 (s, 1H, aromatic CH2); δ

3.63-3.44 (m, 4H, aliphatic CH2); δ 1.15-1.11 (t, 6H, aliphatic CH3);

13C NMR (CDCl3, 300 MHz): δ (ppm) 172.0, 163.0, 158.2, 155.9, 152.2,

144.5, 144.4, 137.4, 131.3, 122.3, 117.5, 112.4, 110.1, 108.0, 95.7, 62.7,

44.3, 12.3.

2.3.2.7 Synthesis of 3-(4-(4-chloro-4-(7-(diethylamino)-2-oxo-2H-chromen-3-

yl)buta-1,3-dien-1-yl)phenyl)-2-cyanoacrylic acid [10b]

The procedure was same as that for compound 10a except that 2-cyano-3-(p-tolyl)

acrylic acid [9a] (0.76 g, 4.0 mmol) was used instead of cyanoacetic acid. Yield =

0.91g (59%); M.P. = 226 °C.

Analysis of dye [10b]:

A. Mass spectra of the compound showed ion peak at m/z = 422 which

corresponds to molecular weight of [10b] after removal of Cl and OH



groups; msms showed ion peak at m/z = 337 and 255 which also matches

with fragment ions after suitable fragmentation.

B. The compound was further confirmed by which showed following signals

[10b].

1H NMR (CDCl3, 300 MHz): δ (ppm) 8.04-7.90 (d, 1H, aromatic CH); 7.90-

7.80 (m, 5H, four aromatic CH and one vinylic CH); 7.50-7.42 (m, 4H, three

aromatic CH and one vinylic CH); δ 6.90-6.80 (m, 2H, vinylic CH); δ 3.72-

3.60 (m, 4H, aliphatic CH2); δ 1.50-1.38 (t, 6H, aliphatic CH3)


142.3, 129.7, 128.8, 123.7, 117.8, 109.5, 108.8, 97.0, 45.1, 12.6

2.3.2.8 Synthesis of 3-(5-(4-chloro-4-(7-(diethylamino)-2-oxo-2H-chromen-3-

yl)buta-1,3-dien-1-yl)thiophen-2-yl)-2-cyanoacrylic acid [10c]

The procedure was same as that for compound 10a except that 2-cyano-3-(5-

methylthiophen-2-yl)acrylic acid [9b] (0.78 g, 4.0 mmol) was used instead of

cyanoacetic acid. Yield = 0.70g, (45%); M.P. = 212 °C.

Mass spectra of the compound showed ion peak at m/z = 199, 244, 407 which

corresponds to molecular weight of [10c] after suitable fragmentation.

2.3.2.9 Synthesis of 8-methoxy-1, 2, 3, 5, 6, 7-hexahydropyrido [3, 2, 1-ij] quinoline

[12]

In a 500ml three-necked round bottomed flask equipped with an overhead

mechanical stirrer, thermometer, and a pressure equalizing addition funnel, 3-

Methoxyaniline (10g, 11ml, 0.10moles) was taken to which 1-bromo-3-

chloropropane (186g, 160ml, 1.5moles) and anhydrous sodium carbonate (42.7g,



0.4moles) were added. The top of the addition funnel was fitted with a condenser

and the reaction mixture was warmed to 70°C for 1 hr and 100 °C for 2 hrs and then

heated at reflux for 11 hrs. The progress of the reaction was monitored by thin layer

chromatography. The reaction mixture was cooled to room temperature and 150ml

of concentrated HCl and 50ml of water were slowly added. Upon dissolution of all

solids, the phases were separated and the organic layer was washed with 10%HCl to

remove remaining product. This washing was added to the aqueous phase, which

was washed with ether to remove 1-bromo-3-chloropropane. The aqueous phase was

made basic with 50% aqueous sodium hydroxide and extracted with ether until the

organic phase no longer colored. The ethereal solution was dried over MgSO4 and

the solvent was removed. The resulting brown oil was distilled under reduced

pressure (0.5-1mm Hg, 110-114°C) to give yellow oil which turned red on exposure

to air. Yield = 13g (64%).

2.3.2.8 Synthesis of 1,2, 3,5,6,7-hexahydropyrido[3,2,1-ij]quinolin-8-ol [13]


mechanical stirrer, 8-Methoxy julolidine (10g, 50 mmol) was dissolved in a solution

consisting of 50 ml of 47% HI, 80ml of concentrated HCl and 200ml of water. This

solution was heated at reflux and the progress of the reaction was monitored by thin

layer chromatography. After 15 hrs, another 50ml portion of concentrated HCl was

added to the reaction mixture. After observing complete consumption of starting

materials (about 60 hrs) by thin layer chromatography, the reflux was stopped. The

solution was cooled in ice bath and neutralized to pH 6 by using NaOH. The

precipitate obtained was filtered to get tan coloured solid.



Yield = 5.9 g (63%); M.P. = 126-130 °C.

2.3.2.9 Synthesis of 8-hydroxy-1,2,3,5,6,7-hexahydropyrido[3,2,1-ij]quinoline-9-

carbaldehyde [14]


mechanical stirrer, compound [13] (5.0g, 26.4 mmol) was dissolved in 8ml of dry

DMF and the resulting solution was added dropwise to cold solution of phosphorus

oxychloride (POCl3) (4.46g, 2.66ml, 29.1mmol) in 10ml of dimethyl formamide

which had been stirring for 15 minutes. The mixture was then stirred for 30 minutes

at room temperature followed by heating the solution at 85-90 °C for 2 hrs. The

reaction mixture was poured in ice-water and neutralized to pH 6-7 using sodium

carbonate solution. The precipitate obtained was filtered, washed with water and air-

dried. The residue was recrystallized from hexane to give yellow crystals.

Yield = 4.5g (78%); M.P. = 70-72°C.

2.3.2.10 Synthesis of 10-acetyl-2,3,6,7-tetrahydro-1H-pyrano[2,3-f]pyrido[3,2,1-

ij]quinolin-11(5H)-one [16]

In a 250ml single necked round bottom flask fitted with reflux condenser, compound

[14] (4.2 g, 0.02moles) was taken along with ethylacetoacetate (2.5g, 0.02moles) in

(42 mL, 10 vol) ethanol and 0.5ml piperidine. The reaction mass was refluxed for 5-

6 hrs and checked for the completion of reaction by thin layer chromatography. The

contents of the flask were cooled to room temperature and the product [15] separated

as orange crystals were filtered, washed with little amount of ethanol and dried.

Yield = 4.1 (76%); M.P. = 181-182 °C.



2.3.2.11 Synthesis of 3-chloro-3-(11-oxo-2,3,5,6,7,11-hexahydro-1H-pyrano[2,3-

f]pyrido[3,2,1-ij]quinolin-10-yl)acrylaldehyde [17]

In a 100ml three necked round bottom flask fitted with a mercury sealed stirrer,

addition dropping funnel was fitted which was topped by calcium chloride guard

tube and reflux condenser also topped by calcium chloride guard tube. In this flask,

N,N-dimethyl formamide (2.32 g, 2.4 ml, 30 mmol) was taken and cooled to 0-5°C

with stirring. Then, phosphorous oxychloride (2.24g, 2 ml, 0.020 moles) was added

drop wise maintaining the temperature of the reaction mass at 0-5°C. The Vilsmeier

complex so formed was stirred for further 15 minutes and [16] (3g, 10.6 mmol) was

added in lots (15-25 minutes) to the complex. The reaction mixture was stirred at 0-

5°C for 3 hrs and then allowed to attain room temperature. The contents of the flask

were heated at 80-85°C in a water bath for 6-7 hrs. Subsequently the reaction mass

was cooled to room temperature and poured in to crushed ice with stirring and deep

external cooling. The clear solution so obtained was neutralized with sodium

carbonate to pH 7-8, by keeping the temperature below 10°C. The brick solid

colored product separated was filtered, washed with ice cold water and dried at

50°C. Yield = 2.1 g (61%); M.P. = 196-198°C.

2.3.2.10 Synthesis of 5-chloro-2-cyano-5-(11-oxo-2,3,5,6,7,11-hexahydro-1H-

pyrano[2,3-f]pyrido[3,2,1-ij]quinolin-10-yl)penta-2,4-dienoic acid [18]

In a 100ml three necked round bottom flask fitted with a mercury sealed stirrer, a

mixture of 3-chloro-3-(11-oxo-2,3,5,6,7,11-hexahydro-1H-pyrano[2,3-

f]pyrido[3,2,1-ij]quinolin-10-yl)acrylaldehyde [17] (1.5 g, 4.5 mmol) and

cyanoacetic acid (0.60 g, 6.8 mmol) was taken in absolute ethanol (15 mL, 10 vol)



to which piperidine (0.5 mL) was added. The reaction mass was stirred and refluxed

for 4 hrs. The progress of reaction was monitored by TLC. After completion of the

reaction, the reaction mass was added to cold water and product was extracted using

ethyl acetate. The ethyl acetate layer was washed with water and then subjected to

evaporation under vacuum using rotary evaporator to obtain product [18]. The crude

product was further purified by silica gel column chromatography using toluene:

ethyl acetate system (6:4) as eluent system. Yield = 0.97 g (54%). M.P. = 236 °C.

Analysis of dye [18]:

A. Mass spectra of the compound showed ion peak at m/z = 297, 269, 261

which corresponds to molecular weight of [18] after suitable fragmentation.

B. The compound was further confirmed by which showed following signals

[18].

1H NMR (CDCl3, 300 MHz): δ (ppm) 8.30 (s, 1H, aromatic CH); 8.18-8.09

(m, 2H, vinylic CH); 7.20 (s, 1H, aromatic CH); δ 2.75-2.60 (m, 4H,

aliphatic CH2); 2.50-2.40 (m, 4H, aliphatic CH2); 1.90-1.75 (m, 4H, aliphatic

CH2)


145.3, 142.5, 127.4, 119.9, 115.4, 109.8, 108.2, 104.5, 49.7, 49.2, 26.7, 20.4,

19.4



In a three-necked round bottomed flask, 3-chloro-3-(7-(diethyl amino)-2-oxo-2H-

chromen-3-yl)acrylaldehyde [4] (2g, 6.5 mmol) was taken in absolute ethanol (20

mL, 10 vol). To this reaction mixture, 3-mercaptopropanoic acid (1.38g, 1.1 mL, 13



mmol)was slowly added followed by addition of triethylamine. The progress of

reaction was monitored by TLC. After maximum consumption of reactants, the

reaction mass was added to cold water and product was extracted using ethyl acetate.

The ethyl acetate layer was washed with water and then subjected to evaporation

under vacuum using rotary evaporator to obtain product [21]. The crude product was

further purified by silica gel column chromatography using toluene:ethyl acetate

system (7:3) as eluent system. Yield = 1.27 g (52%)



In a three-necked round bottomed flask, malononitrile [22] (0.3 g, 0.25 mL, 4 mmol)

was taken in absolute ethanol (10 mL, 10 vol) to which piperidene (0.1 mL) was

slowly added. The temperature of the reaction mass was increased to 50 °C. This

was followed by the slow addition of 3-((1-(7-(diethylamino)-2-oxo-2H-chromen-3-

yl)-3-oxoprop-1-en-1-yl)thio)propanoic acid [21] (1g, 2.6 mmol) and the reaction

mixture was vigorously stirred at reflux temperature for 5 hrs. The progress of the

reaction was monitored by TLC. After completion of the reaction, the reaction mass

was added to cold water and product was extracted using ethyl acetate. The ethyl

acetate layer was washed with water and then subjected to evaporation under

vacuum using rotary evaporator to obtain product. [23]. The crude product was

further purified by silica gel column chromatography using toluene:ethyl acetate

system (7:3) as eluent system. Yield = 0.7 g (64%). M.P. = 266 °C. Mass spectra of

the compound showed ion peak at m/z = 199, 244, 407 which corresponds to

molecular weight of [23] after suitable fragmentation.



2.4 CONCLUSION

In this chapter, we had aimed at designing coumarin based novel colorants suitable

for application in dye-sensitized solar cells. On varying the conjugation bridge in the

colorants, it was observed that UV-visible absorption gets bathochromically shifted.

In this regards, the thiophene bridged dye gave broader absorption in comparison to

other dyes. The fused coumarin dye also gave a bathochromic shift but its absorption

was not very broad. The concept of lateral chain anchoring seemed to improve

charge separation in the dye and gave a broad absorption till 600 nm.

The coumarin based dyes, designed and synthesized in this chapter is further studied

for its application in dye-sensitized solar cells as discussed in chapter 3. The

electronic transfer states of coumarin dye are also known to be affected by the open

chain or fused amino group. This aspect is also covered in chapter 3 by conducting

ultrafast laser studies on some dyes synthesized in this chapter.



N O O

C l

CN

COOH

Mass spectra



N O O

Cl

H3C

H3C

CN

COOH



1H-NMR spectra



with increased intensity

chapter 2: design and synthesis of coumarin based...

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